1. Field of the Invention
The present invention relates to a liquid crystal display device. More specifically, the present invention relates to an active matrix liquid crystal display device in which each pixel electrode arranged in a matrix form is selectively driven with active elements.
2. Description of the Related Art
A liquid crystal display device is necessary as a means for displaying information or pictures, and recently, active matrix liquid crystal display devices have been widely used in applications such as a TV and a monitor. In such applications, a liquid crystal display device is illuminated for a long time and further used over a long period of time, leading to a desire for further improvement in reliability for long term use.
In an active matrix liquid crystal display device, the device is greatly influenced by voltage retention, and thus studies have been made for sufficient elimination of partial unevenness of luminance, spots, etc. caused by lowering of voltage retention by impurity ions within the panel.
Conventionally, in order to prevent display defects from being caused by elution of impurity ions to a display region in a liquid crystal display device, dummy electrodes or dummy pixels are arranged in a non-display region and a voltage is applied to the dummy pixels to retain impurity ions of the non-display region section within the non-display region and to improve reliability (refer to e.g., Japanese Kokai Publication Hei-04-295824 (pgs. 2-3, FIG. 1) and Japanese Kokai Publication Hei-09-5780 (pgs. 2, 5, FIG. 1)).
A mechanism of occurrence of unevenness of display and spots caused by impurity ions and conventional liquid crystal display devices will be described. FIG. 7A is a plan view of a panel of a conventional liquid crystal display device. And, FIG. 7B is a cross sectional view of a part of a display region 21 and a non-display region 22 of the panel cut along the cross sectional line F-F′ of FIG. 7A. As illustrated in FIG. 7A, in conventional devices, the dummy pixel 24 driven in the same way as a dummy electrode or a pixel is arranged in the non-display region 22 between the display region 21 and the seal 23.
In such liquid crystal display devices, the non-display region on the inner side of a seal consists of (I) dummy pixels or dummy electrode sections paired by a combination of a first substrate and a second substrate, (II) a BM (black mask) stripped section of the second substrate and an insulating layer section of the first substrate placing opposite the BM stripped section, and (III) a seal/liquid crystal boundary section. Impurity ions used in the BM material, and impurity ions attached to the BM surface that were not washed off are eluted from (II). The display region has an orientation film printed on the BM, resulting in a slight influence by elution of impurity ions.
Also, impurity ions that have not reacted during hardening of the seal are eluted from (III).
These impurity ions enter a display region to cause a reduction in voltage retention, resulting in the occurrence of unevenness of luminance, spots, etc.
In order to prevent the above-described problems from occurring, conventionally, impurity ions are retained within the region of (I) by driving a dummy electrode or a dummy pixel and applying a voltage thereto, thereby the influence on a display region is prevented.
However, in the above-described conventional art, the following two points need to be further improved.
(1) Elution of impurity ions within a non-display region is accelerated.
(2) Impurity ions retained within a dummy electrode or a dummy pixel enter a display region when power is turned off, which results in causing unevenness of luminance and spots.
Regarding (1), an electric field is generated in a non-display region under the influence of driving a metal wiring, a dummy electrode or a dummy pixel of a first substrate, and the electric field draws out impurity ions attached to a BM of the non-display region and impurity ions used in the BM or a seal itself, resulting in acceleration of elution of impurity ions.
Next, regarding (2), conventionally, eluted impurity ions are retained within a non-display region by application of a voltage to a dummy electrode or a dummy pixel, but when power is turned off, the retained impurity ions are eluted to a display region.
As is mentioned above, a sufficient reliability may not be obtained when liquid crystal display devices are used for TVs, etc. that are used over a long period of time and turned on and off many times, by a method of trapping impurity ions with a dummy electrode or a dummy pixel as in conventional liquid crystal display devices, and thus, a sufficient elimination of unevenness of display, spots, etc. caused by reduction of voltage retention by impurity ions, and further improvement in reliability of long time/long term use in active matrix liquid crystal display devices, etc. are desired.